1. Field of the Invention
This invention relates to a method and device for processing signals representative of waves reflected, transmitted or refracted by a volume structure with a view to exploring and analyzing said structure.
It applies notably, though not exclusively, to the manufacture of equipment such as echo sounding apparatus, non-destructive testing instruments, sonar or even radar equipment.
2. Description of the Prior Art
Conventional instruments of this type usually use transmission means which transmit an incident wave into the medium or environment to be examined, and reception means, that may use all or part of the transmission means (homodyne systems), which receive the incident waves reflected by the structures encountered by the incident wave. Another means is also provided to transform and process the signals received by the reception means and to present them in a form that can be used by the user, e.g. in the form of an image enabling location of the position of the obstacles causing the incident wave to be reflected.
The method most widely used to obtain these results consists in using pulsed waves according to a process consisting in transmitting a pulse in a given direction (shot), in detecting the return of the echoes, in measuring the time lapsed between the transmission and reception and in deducing the distance and therefore the position of the obstacle that generated each echo. This shooting process is repeated for different directions, according to a predetermined scanning law.
Once this scanning has been performed, it then become possible to produce images, e.g. on a conventional display system, showing the obstacles detected by the echoes and whose positions are now known.
Numerous instruments of this type use the so-called "sequential" method by way of which the structure is examined line by line by means of a mobile beam, the exploration line being moved between shots.
In these conditions, the speed of examination increases with the cross-section of the exploring beam and the pulse rate. However, it so happens that the cross-section of the beam is limited by the spatial resolution required, whereas the pulse rate is limited by the time needed for all the reflected echoes to return to the probe.
To take an example, in order to examine an aluminum plate in which we are seeking to detect flaws of 1 mm in diameter, with a resolution of 3 mm, the cross-section of the beam may hardly exceed 2 mm, and the pulse rate must be below 1000 Hz in view of the reverberation phenomenon.
In these conditions, the surface examination speed cannot exceed 2 mm.times.2 mm.times.1000=4000 mm.sup.2 per second, i.e. 4/1000ths of a square meter, this being the equivalent of 4/1000ths.times.3600=14.4 m.sup.2 per hour. At the end of the production chain, this speed is often too slow insofar as it slows down production, whence the need to operate several installations in parallel.
In numerous other applications (pipe checking, rails on track, etc.), this limitation is even more critical.
With a view to remedying these drawbacks, it has already been proposed that there be transmitted, towards an object to be explored, a substantially plane wave of relatively large cross-section and generated by a probe constituted by a network comprising a plurality of transmission/reception units of small size, preferably less than one wavelength, in order to have a very large radiation pattern; these transmission units being attacked simultaneously, in parallel. From the point of view of reception, each transmission/reception unit operates independently, therefore receiving separately the waves reflected by the obstacles intercepting the beam located in its reception zone. After digitization, the data supplied by these transmission units (field of reflected waves) are stored in memories which are read in the opposite direction to that of write operations therein.
The read signals are then applied to a device for reconstituting the field of reflected waves, said device comprising a plurality of transmission units distributed according to a structure similar to that of the transmission/reception units of said probe. The read signals are applied to these transmission units in correspondence with the transmission, to the memory, of the write signals by the transmission/reception units.
The purpose of the reconstitution device is to reproduce, in an auxiliary environment, the reflected wave field in order to reproduce an image of the object, with a resolution which depends on the wavelength of the incident wave and on the dimensions of the probe elements.
In the case of the incident wave being an ultrasonic wave, the simplest solution is to form the image in an optically transparent environment and to view it by Schlieren's method.
However, this method does not readily lend itself to industrial use. Moreover, it is not linear and does not enable the rendering of high frequency components.
According to another method, the image is received on a third probe and the reading frequency is modulated so that the image of a structure is also focused when the corresponding signals arrive on this probe.
Experience has proved this system to be complex and that it requires probes with a very wide band. Furthermore, the signal becomes deteriorated after passing through three successive probes. In addition, further difficulties arise when the transmission wave is curved or circular.